Radio transmitter power amplifiers, like any electrical components in the radio transmitter chain, can produce distortion in the signal to be transmitted. An undesirable byproduct of the distortion produced by a radio frequency power amplifier is spectral regrowth. FIG. 1(A) illustrates an example of a desired transmit channel frequency band in which the originally transmitted signal should be contained. However, as a result of distortion in the radio power amplifier, spectral regrowth occurs as shown in FIG. 1(B) with a considerable portion of transmitted power falling outside of the desired or permitted transmission band for that radio.
Such spectral regrowth is inherent in spectrally efficient linear modulation techniques such as quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), and .pi./4 differential quadrature phase shift keying (.pi./4-DQPSK). These modulations are generally very spectrally efficient providing a high ratio of data rate per frequency spectrum used. In general, linear modulation is any modulation which produces changes in the amplitude of the output signal in contrast with angle modulation (e.g., frequency modulation or phase modulation) where only the phase of the signal varies and the amplitude remains constant. Unfortunately, non-linearities in the signal path (produced for example by the power amplifier) cause problems for linear modulation formats in the sense that the transmitted frequency spectrum spreads out and interferes with transmissions on adjacent radio channels.
Cartesian feedback loops may be used to linearize one or more amplifiers of the transmitter amplifier chain. The term "linearize" is used here in the sense that feedback is used to reduce or eliminate distortion products generated or caused by the transmitter amplifier chain. In Cartesian feedback linearization, the radio transmitter power amplifier output is detected, downconverted from radio frequency, and broken down into its in-phase (I) and quadrature (Q) components by an I/Q demodulator. The downconverted I and Q components are then subtracted from the input I and Q signals leaving two residual error signals. These error signals contain the unwanted distortion terms generated by the power amplifier chain. Negative feedback is used to reduce these error signals hopefully to zero.
The term "Cartesian feedback" is derived from quadrature modulation. In quadrature modulation, there are two independent orthogonal modulating input signal referred to as the in-phase (I) and quadrature (Q) signals modulating onto a single carrier. The I signal is multiplied by a cosine wave at the carrier frequency, and the Q signal by a sine wave carrier. The resultant signals are added to form the composite I/Q modulated signal. Quadrature or I/Q modulation may therefore be viewed as the vector addition of two orthogonal signals, a real component representing the x-axis component in a Cartesian coordinate system and an imaginary component representing the y-axis component. Thus, the similarity of I/Q modulation in the complex domain to two-dimensional Cartesian coordinates gives rise to the name Cartesian feedback.
Radio frequency power amplifiers produce not only distortion products but also phase shifts because of finite time delays in signal propagation. Phase shifts are also caused by cables (e.g., approximately 45.degree. of phase shift per inch of cable for a 1 GHz signal), filters, power splitters/combiners, and almost an component associated with coupling RF signals to and from a power amplifier. One difficulty with these phase shifts is that they are generally unknown and are affected by temperature shifts, frequency shifts, power supply drift, and other factors that are generally variable and difficult to predict.
Such phase shifts are particularly problematic in a feedback-based system. In general, uncompensate phase shifts degrade the performance of the feedback loop causing excess noise output, spurious outputs, and/or system instability. More specifically, in a Cartesian feedback loop, if the phase shift were as much as 90.degree., the demodulated in-phase component (I) would be "replaced" by the demodulated quadrature component (Q). As a result, none of the correct signal would be fed back rendering the feedback loop completely inoperative. In practice, even phase shifts of 15.degree. or more are difficult to tolerate.
One approach to dealing with these undesired phase shifts is to initially try to measure the phase shift in a calibration routine and introduce the opposite phase shift detected during the calibration routine into the loop using, for example, a digital phase shifter. However, this approach would be disadvantageous in that the calibration routine is time consuming and assumes that the phase shift is completely constant over time and environmental variables. This assumption is not correct, and thus, the approach would not compensate for the actual phase shift in real time. Moreover, digital phase shifters have limited resolution which limits the accuracy of the phase compensation. Another difficulty with this approach is presented each time the radio is modified, e.g., a transmitter board, power amplifier, cable, etc. is replaced, requiring recalibration.
It is an object of the present invention to provide a method and apparatus for compensating for unwanted phase shifts without suffering the disadvantages and limitations note above. For example, it is an object of the present invention to continuously track and cancel unwanted phase shifts in a radio transmitter chain using a technique and circuitry that is inherently self-calibrating, adaptive, and highly accurate. In a feedback-based transmission system, it is another object of the present invention that such phase shift tracking and cancelling techniques and circuitry do not adversely impact the transmitter feedback loop.
In the context of a feedback based radio frequency transmission system, the present invention provides a method for substantially continuously tracking and cancelling in real time unwanted phase shifts caused by the radio frequency transmitter chain. Initially, a carrier signal is modulated with a difference or error signal based on an input signal and transmitted. The transmitted signal is detected and demodulated to generate a feedback signal. The difference between an input signal and the feedback signal is determined to generate the difference signal. The phase of the detected feedback signal is substantially continuously compensated for undesired phase shifts.
A radio transmitter in accordance with the present invention includes a local oscillator for generating a local oscillator signal, a modulator for modulating input signals onto the local oscillator signal to generate a forward signal, a power amplifier for amplifying the forward signal, and an antenna or other transmission medium for transmitting the amplified signal. A coupler, power splitter, or other sampling device is used to detect the transmitted signal. A quadrature demodulator demodulates the detected signal in accordance with the local oscillator signal whose phase has been adjusted by a phase adjuster connected to the local oscillator and the detector. Demodulating the detected signal using the phase adjusted local oscillator signal effectively cancels undesired phase shifts in the quadrature demodulator detected signal.
In the example embodiment of the present invention as applied to a Cartesian feedback system, the modulator is a first quadrature modulator for modulating in-phase (I) and quadrature (Q) signals. The demodulator is a quadrature demodulator that generates feedback in-phase and quadrature signals that are combined with the input in-phase and quadrature signals in corresponding combiners. Thus, the in-phase and quadrature signals provided for the quadrature modulator are difference or error signals generated by the feedback loop.
The phase adjuster in the example Cartesian feedback embodiment includes a second quadrature demodulator. The forward signal is connected to one input port of the second quadrature demodulator, and the sampled or feedback signal is connected to another input port. The second quadrature demodulator generates second set of in-phase and quadrature signals which carry information about the phase shift of the incoming feedback signal. The peak amplitude of these signals are detected by corresponding peak detectors. A second quadrature modulator receives at its in-phase and quadrature input ports the output signals from the first and second peak detectors as well as the local oscillator signal and generates a phase adjusted local oscillator signal.
The second demodulator can be viewed in this example embodiment as a four quadrant phase comparator with the unknown phase shift between the forward and detected signals appearing at its in-phase and quadrature outputs in the four quadrant complex phase plane similar to the Cartesian coordinate system as described above. Since these I and Q outputs are modulated in amplitude by the signal modulation, the peak detectors remove the amplitude modulation components leaving only phase shift information. Mathematically, this is given by EQU (unwanted phase)=arc tan (Q/I),
where Q=output of peak detector "Q" and I=output of peak detector "I". The undesired phase shift information from the detected signal is "passed on" to the second I/Q modulator at its I and Q inputs so that the phase of the local oscillator input to the primary I/Q demodulator in the feedback loop is identical to the phase of the detected signal. With identical phases in the local oscillator and the feedback signal, the original I and Q signals (plus distortion terms) are recovered without cross talk and with correct polarity. As a result, the unwanted phase shift or difference is effectively cancelled.
Another application of the present application for detecting and compensating for undesired phase shift is found in diversity radio receivers. In a diversity radio receiver, two or more radio receivers demodulate signals received from two or more separate antennas. The "best" signal is chosen, e.g., on the basis of greatest received signal strength indicator level (RSSI level), and the best modulated signal is passed to the next radio receiver stage, e.g., audio to a speaker, data stream to a data decoder, or a modulator/upconvertor in the case of a repeater or base station.
Diversity reception is a common technique used to reduce the effects of fading in terrestrial radio systems. When one of the antennas temporarily receives a weak or corrupted signal, typically due to destructive cancellation from multi-path signal reflections, another antenna likely receives a stronger and/or clearer signal. Diversity reception is particularly advantageous for receiving signals from mobile transceivers which are used in a constantly changing environment where the occurrence of signal fading is both frequent and unpredictable.
Diversity receivers, as normally implemented, exhibit most improvement over standard receivers using only a single antenna when one incoming signal is much stronger than another. But when two signals arrive at a dual input diversity receiver at approximately the same signal level, there is little or no signal to noise ratio improvement from switching between signals. So diversity switching offers little improvement in this situation. Some diversity systems blend or mix received demodulated audio signals to obtain better performance under the same-signal conditions, but the increase in signal to noise ratio is limited to 3 dB at most.
The optimum way to use both signals to improve signal to noise ratios is to add the two signals before demodulating. Additive signal combination before demodulation under same-signal conditions provides at least 3 dB of signal to noise ratio improvement, and even more closer to the FM threshold where a small change in input signal integrity results in a large change in demodulated signal to noise ratio. But before the two diversity signals can be added effectively, they must be aligned in phase to prevent destructive interference caused by oppositely phased signals. In addition, such phase alignment must be done dynamically, since the unknown phase difference between the two signals will change with time as relative movement occurs between the transmitting radio and the receiving radio having the diversity receive antennas.
It is an object of the present invention therefore to provide a method and apparatus for compensating for such phase shifts between the two diversity receiver signals. Moreover, it is an object of the present invention to dynamically phase align diversity signals to allow for in-phase combination that results in, among other things, improved signal to noise ratio under same-signal reception conditions.
The first of two diversity antennas passes an RF received signal to the front end of a first diversity receiver which downconverts the received signal to an intermediate frequency. The same down conversion is performed in the front end of the second diversity receiver. For the reasons explained above, the two IF signal have an unknown, changing phase difference. These two IF signals are processed by an I/Q demodulator which, in conjunction with two peek detectors/low pass filters at the I/Q outputs, function as a four quadrant phase comparator. The peak detector outputs provide I and Q inputs to I/Q modulator which receives as its local oscillator input the received IF signal from one of the first and second diversity receivers. The I/Q modulator functions as a phase shifter, rotating the phase of the one received signal to match the phase of the other received signal. After the phase rotation, the two IF signals are than additively combined and routed to main radio demodulator.